Teaching another person, even an eager one, how to change their behaviour and mental patterns concerning validation and FOMO is a really hard task. It's not knowledge like math or something that you can just pass onto them. It can take months or even years even for pros (psychitrists etc.) to do that. Combine that with the fact that teenagers brains are wired differently (their need to fit it is greater than that of an adult) and their "natural resistance" to adults' teaching and it becomes almost impossible task.
Ever tried teaching anything to teenagers where their peers and society in general promote the opposite? Good luck with that...
Like a strict parent whose kids end up doing drugs and partying as soon as they can leave the house, or even the opposite, some "hippie" all-too-liberal parent whose kind grow up and seek strictness and discipline, and e.g. turn religious or join a cult, or find some abusive partner who gives that to them.
Social media can be really bad, and maybe it's actually mostly bad, but it does facilitate real life interactions and good things.
Imagine your child going to school but not being allowed to play with other kids in recess. Lots of bullying and physical violence can happen during recess, after all. Would you use that as an opportunity to teach them the dangers of FOMO, and to just read a book?